Introduction
The periodontium – gingiva, periodontal ligament, root cementum and alveolar bone – attaches the tooth to the jaw. In many European textbooks the whole apparatus is the parodontium, and periodontium in the narrow sense is the periodontal ligament (desmodontium). Its inflammation around the apex – apical periodontitis – nearly always comes from an infected root canal and is the main reason for root canal treatment.
Anatomy and histology
- Gingiva – masticatory mucosa around the tooth neck, attached by the junctional epithelium and gingival fibres; it seals the deeper tissues from the mouth.
- Cementum – avascular mineralised tissue (about 45–50% mineral) that anchors the ligament fibres. Acellular (primary) cementum covers the cervical third to half of the root; cellular (secondary) cementum, with cementocytes, covers the apical part and furcation and is laid down throughout life.
- Alveolar bone – the socket is lined by the alveolar bone proper, a thin compact plate perforated by vessels (cribriform plate) in which the fibres insert as Sharpey's fibres; behind it lies cancellous bone. The crest lies about 1.5–2 mm below the CEJ.
On a radiograph the alveolar bone proper is the lamina dura: a thin continuous radiopaque line beside the uniform radiolucent PDL space. A widened apical PDL space and a broken lamina dura are the earliest signs of apical periodontitis.